1. Field of the Invention
The present invention relates to a probe for a near-field microscope in which a probe is caused to come close to or contact with an evanescent field generated on a sample surface, an evanescent light is scattered by a probe and, by detecting the scattered light by a photodetector, a local optical characteristic of the sample is measured with a resolution beyond diffraction limit of the incident light.
2. Description of the Related Art
In a conventional optical microscope, a spatial resolution has been limited to a length of about half of a used wavelength by the diffraction limit. However, in recent years, it becomes that a development in a technique called nano-technology is eagerly performed, and there is an increased demand for measuring the optical characteristic of a substance with the resolution exceeding the diffraction limit.
In order to realize the demand, a development in the near-field probe microscope is eagerly performed.
The conventional near-field probe microscope is classified into a fiber type and a scattering type.
In the fiber type near-field microscope, a tip of an optical fiber is sharpened, the size of an aperture is 100 nm or less, and portions other than the opening part is shielded from the light by a metal. When a laser is entered from an optical fiber aperture, the evanescent light is produced in the vicinity of the aperture. A probe approaches a sample by utilizing a shear force or an atomic force, which acts between a probe tip and the sample surface, the evanescent light irradiate the sample by measuring a near-field light intensity or a spectrum by the photodetector, the optical characteristic of the sample surface is measured with the resolving power exceeding the diffraction limit.
On the other hand, in the scattering type near-field microscope, the evanescent field is generated on the sample surface, the scattered light is generated by inserting the probe of a metal or a dielectric. The probe is inserted into the evanescent field and, by measuring the near-field light intensity or the spectra by the photodetector, the optical characteristic of the sample surface is measured with the resolving power beyond the diffraction limit.
The fiber type near-field microscope is the present measurement tool, the device is comparatively stable “it evaluates if the level of the background light is low”However, it is used in a fluorescence spectral analysis in which an excited light intensity is weak and a signal light intensity is comparatively easily obtained, an absorption measurement and the like, because a loss of the light in a taper portion inside the optical fiber cause the probe aperture to enlarge.
On the other hand, the scattering type near-field microscope can be used also in a Raman spectral analysis in which the scattering cross-sectional area is small and the signal light is difficult to detect and a nonlinear spectroscopy analysis, because it is possible to increase a incident light intensity imposed on the probe made from the metal or the dielectric and an electric field was enhanced by the interaction of the evanescent field both with incident light and the scattered light.
The scattering type near-field microscope of the prior art is explained on the basis of a schematic view of FIG. 9 (for example: Norihiko Hayazawa, Yasushi Inouye, Zouheir Sekkat, Satoshi Kawata, Near-field Raman scattering enhanced by a metallized tip, Chemical Physics Letters, 335, 369-374, 2001 (FIG. 1)).
A cantilever 101 is used for an interatomic force microscope, which has in its tip a probe 102 of a size of 40 nm in diameter and has been coated by silver 40 nm in thickness.
Further, by setting an objective lens 105 whose numerical aperture is 1.4 in a back side with respect to a measured face of a sample 103 through an oil-immersion oil 104 and entering an annular laser light 106 into a region in which the numerical aperture component of the objective lens 105 exceeds 1, the evanescent field is formed in a surface of the sample 103.
Next, the probe 102 is contacted with the evanescent field generation region of the sample 103 surface while performing a distance control by the interatomic force acting between the probe 102 and the sample 103 surface.
At this time, the evanescent field is scattered by the probe 102. By condensing this scattered light 107 by the same objective lens 105 (not shown in the drawing), an analysis of the local optical characteristic by the probe tip becomes possible.
In this prior art, rhotamine 6G that is one kind of dye is measured as the sample. In the scattered light from the sample Raman scattered light is generated also besides a Rayleigh scattered light whose wavelength is the same as the excited light.
Further, by the facts that a surface plasmon is excited in a surface of the silver coated probe 102 and that the sample 103 and the silver probe 102 tip are contacted, a so-called surface enhanced Raman scattering (SERS: Surface Enhanced Raman Scattering) occurs, so that it becomes possible to enhance the Raman scattered light.
By condensing these scattered lights by the objective lens 105, removing the Rayleigh scattered light by a notch filter or the like and, after spectrally dispersing them by a spectroscope, detecting it by a liquid nitrogen cooled CCD, it is possible to obtain a local Raman spectra (the notch filter, the spectroscope and the CCD are not shown in the drawing).
An electron microscope photograph of the probe 102 provided in the cantilever 101 used in FIG. 9 is shown in FIG. 10.
The fact is seen that a silver particle of the probe 102 surface adheres to a probe surface like an island and has a longitudinally long shape. Further, in a tip portion 102a of the probe, the whole tip portion of the probe is covered by a uniform silver particle.
Next, FIG. 11-FIG. 13 and Table 1 are explained about the Raman spectral analysis.
Since a vibration spectroscopy mainly such as Raman spectroscopy can obtain a structural information in comparison with the fluorescence spectral analysis and the absorption measurement, it is possible to obtain detailed information concerning molecule vibration, orientation, intermolecular interaction, and excited state. However, since the Raman scattered light is generally very weak, its measurement is difficult. Especially, in a case where the excitation is performed by the evanescent light, the light quantity becomes weaker.
Therefore, in a case where the Raman spectroscopy is performed by scattering the evanescent field by the probe, a Raman signal is intended by utilizing the surface enhanced Raman scattering like the prior art mentioned above.
The surface enhanced Raman scattering is a phenomenon in which, by an electron excitation (surface plasmon) of a metal surface, a Raman scattering cross-section area concerning of the signal light from the metal surface is enhanced to about 105 to 1010 times.
Accordingly, in order to efficiently induce the surface enhanced Raman scattering, a condition for efficiently exciting the surface plasmon is necessary.
And, an efficiency of the surface plasmon excitation largely depends on a kind of the metal and a size and a shape of the particle.
As to the metal, it is known that, in a visible light region silver is efficient.
Table 1 shows a value of an imaginary number part of a permittivity at a plasmon absorption maximum wavelength and an absorption maximum of a representative metal (refer to C. F. Bohren, D. R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley 1983 (Table 12-1)).
TABLE 1BulkSurfaceplasmonplasmonPermittivity: ε = ε′ + i ε″energyenergy(real part ε′ = value of imaginaryMetal kind(eV)(eV)number part ε″ at −2)Lithium6.63.41.0Sodium5.43.30.12Potassium3.82.40.13Magnesium10.76.30.5Aluminum15.18.80.2Iron10.35.05.1Copper—3.54.9Silver3.83.50.28Gold—2.55.0Graphite—5.52.7
From Table 1, among the metals having the absorption maximum in the visible light region, silver has the imaginary number part of the permittivity which is peculiarly small. This shows the fact that an attenuation of the plasmon is small and accordingly the silver is most excellent for the surface enhanced Raman scattering in the visible light region.
Further, an influence of a radius of curvature exerting on the absorption cross-sectional area of the silver is shown in FIG. 11, and an influence of the radius of curvature exerting on a near-field scattering efficiency in FIG. 12 (refer to S. Kawata ed, Near-Field Optics and Surface Plasmon Polaritons, Topics Appl. Phys. 81, 97-122 (2001) (FIG. 7, FIG. 8)).
From FIG. 11, it is apparent that the absorption cross-sectional area of the silver has a peak when a particle size is between 10 nm and 50 nm in radius, and becomes a maximum value when the radius is 10 to 20 nm. Since a light energy absorbed contributes to a plasmon excitation, the fact that the absorption cross-sectional area is large becomes the fact that an energy inducing the surface enhanced Raman scattering is large. Further, from FIG. 12, also a near-field scattering efficiency has a peak in a vicinity where the particle size is 10 nm to 50 nm in radius and becomes maximum when the radius is 20 nm, and in this region it is possible to efficiently scatter a near-filed light.
Additionally, in FIG. 13, there is shown a plasmon absorption efficiency depending on a shape of the silver particle (refer to Stockle R M, Deckert V, Fokas C, Zenobi R, Controlled formation of isolated silver islands for surface-enhanced Raman scattering, APPLIED SPECTROSCOPY 54 (11): 1577-1583 November 2000 (FIG. 2)).
Under a state that the silver particle has flatly adhered to a substrate, although the plasmon absorption is scarcely seen, if an island shape of the silver approaches a spherical shape by annealing the surface, a strong plasmon absorption appears. From this fact, for a plasmon induction it is desirable that the shape of the silver particle approximates to the spherical shape, not only its size.
Accordingly, in a case where the Raman spectral analysis is performed by using the near-field, by controlling the shape of the metal particle coated to the probe to a shape approximating to the spherical shape whose radius is 10 nm to 50 nm, it becomes possible to efficiently generate the surface plasmon and, as a result, the Raman scattered light is enhanced and it becomes possible to improve the sensitivity of a Raman spectroscopy. Additionally, by controlling the shape and the size of the metal particle, a quantitative experiment of a surface enhanced Raman scattering effect becomes possible. As a result, it becomes possible to quantitatively estimate the near-field Raman scattered light.
However, in the prior art, since there has not existed a method capable of controlling the shape and the size of the metal particle to optimum ones only by controlling a film thickness of the metal, which is represented by the silver, the gold and the like, coated to the probe, there was no reproducibility with respect to the shape or the size of the metal particle. Further, since an affinity between a substrate and a vapor-deposited metal is good, the vapor-deposited metal forms an island-like film in the vapor deposition object, so that it has been difficult to form a particle-like film.
For this reason, the surface enhanced Raman scattering effect in every probe largely differs, so that it has been impossible to perform from a Raman spectroscopy a quantitative analysis of a intensity.